The invention relates to a method for the acquisition of a spectrally resolved two-dimensional image by means of Fourier transform (FT) spectroscopy, in particular Fourier transform infrared (FTIR) spectroscopy, wherein light is fed to an interferometer and split into two partial beams, wherein the optical path difference between the two partial beams is changed and traverses an identical range several times, and wherein the light is detected by a two-dimensional array detector.
Such a method is known from DE 101 44 214 C1.
In Fourier transform (FT) spectroscopy, light is split into two partial beams, a path difference between the partial beams is impressed, and a detector on which the superimposed partial beams fall is read out. Manipulation of the partial beams is effected in an interferometer, typically with a semi-transparent beam splitter and one or two moving reflectors. Read-out of the detector is repeated for different path differences (repeated read-out at what are known as “scan points”).
Superimposing the partial beams causes interference which, depending on the path difference and on the frequency of the light, can lead to destruction of or high irradiation intensities on the detector. The intensity data of the detector dependent on the path difference is subjected to a Fourier transform, whereby the spectrum of the light is maintained.
FT spectroscopy can be carried out with two-dimensional spatial resolution. In this case, the light of a scene (which is illuminated by way of an interferometer or which emits light into an interferometer) is typically imaged onto a two-dimensional array detector. For each detector element of the array detector the spectrum of the light impinging thereon can be determined by Fourier transform, whereby a spectrally resolved, two-dimensional image is obtained.
For acquiring a single, spectrally resolved image, the optical path difference is usually continuously altered, whilst the detector is read out repeatedly (known as “rapid scan”). For that purpose, a moving reflector of the interferometer is moved by a motor at a constant speed.
Whereas the exposure time of the detector for a scan point is relatively short (typically around 100 μs), read-out and further processing of the data of the individual detector elements requires a considerable time (typically of the order of a millisecond), resulting in a minimum time interval for reading out scan points. When the scan points, which are successive with respect to the optical path difference, are to be read out in succession, the speed at which the reflector can be moved is accordingly limited, or more generally the speed at which the optical path difference can be changed is limited.
In practice, it has been shown that an interferometer cannot be protected entirely against vibrations. These vibrations falsify the actual path difference, impair the spectral resolution of the image acquisition (interferograms “spread” in the time domain) and make the control of the optical path difference (mirror movement) difficult. The vibrations become noticeable above all at a low speed of the change in the path difference, so that a high speed of the change in the path difference is invariably to be preferred.
DE 101 44 214 C1 proposed measuring the scan points needed for an acquisition of a spectrally resolved, two-dimensional image distributed over multiple passes of the optical path difference. In each pass, only some of the scan points needed are read out, wherein in each case one or more scan points in respect of the sequence of the optical path difference are skipped.
This procedure enables the speed of the change in the optical path difference to be kept high in the individual passes, and a good spectral resolution can be achieved. Nevertheless, in this procedure a relatively large amount of time can elapse between the reading out of scan points that follow one another in respect of the optical path difference, since they are measured in different passes. In this period the scene, the spectrally resolved image of which is being acquired, can change noticeably; in particular, objects to be resolved spectrally may move. As a result, during the acquisition of the individual interferograms the detector elements each receive radiation of different objects or object parts, whereby often (if the object movements are too strong) overall it is impossible to obtain any meaningful spectral information.
It is also known to acquire two-dimensional, spectrally resolved images with a single-element detector, wherein the field of view of the system is changed by means of a mirror system after acquiring an interferogram, in order in this manner to scan the image (“scanning spectrometer”).
The invention addresses the problem of providing a method for the acquisition of two-dimensional, spectrally resolved images in which the influence of vibrations on the measurement is reduced, and which is less affected by the movement of objects to be resolved spectrally.